1. Field of the Invention
The invention relates in general to a method of channel estimation, and more particularly to a method of channel estimation which requires a small amount of memory and resources.
2. Description of the Related Art
The orthogonal frequency division multiplexing (OFDM) system having an advantage of preventing multi-path interference resulting in the ghost image is adopted by DVB-T (Digital Video Broadcast Terrestrial) for a high image quality.
In the OFDM system, multi-carrier modulation is performed at the transmitter side and the data are transmitted on sub-carriers through a number of sub-channels. The sub-carriers have different frequencies and are orthogonal to each other such that the data are transmitted at a lower rate in each sub-channel. The characteristics of the transmission channels are time and frequency-dependent. Therefore, the channel response of each sub-channel should be estimated at the receiver side for compensation of the received signals.
Generally speaking, a signal Yki received via a kth sub-channel at time slot ti can be denoted by:Yki=Hki·Xki+Nki  (1)
Wherein Xki is the signal transmitted via the kth sub-channel at the time slot ti, Hki is the channel response of the kth sub-channel at the time slot ti, and Nki is the noise of the kth sub-channel at the time slot ti.
The channel response Hki could be derived by pilot-based channel estimation. FIG. 1 shows a pilot pattern of the OFDM system. Each circle denotes data transmitted by a sub-channel C(0), C(1), . . . , or C(n) at a time slot t0, t1, . . . , or tn. Each of the OFDM symbols S(t0), S(t1), . . . , and S(tn) including a number of signals modulated in one of the sub-channels C(0), C(1), . . . , and C(n) is received at each of the time slots t0, t1, . . . , and tn. The black circle denotes a pilot symbol, and the content and allocation thereof are already known at the receiver side. Therefore, the channel response of each sub-channel could be estimated using the received pilot symbols.
In the estimation of channel response, the influence of noise Nki could be ignored and the estimated channel response could be derived by:Ĥki=Yki/Xki  (2)
Once the channel responses for the pilot symbols are derived, those for the data symbols could be estimated by linear interpolation. The linear interpolation includes a time-domain interpolation and frequency-domain interpolation. FIG. 2 is a flowchart of a method for estimation of the channel response H12 of a sub-channel C(1) at a time slot t2. The channel response Hki is denoted by Aki*exp(jθki), wherein A is the amplitude and θ is the phase. In step 201, the amplitude and phase of the response H32 is obtained. Since the ratio of the interval between the time slots t2 and t1 to that between the time slots t2 and t5 is 1:3 and the linear interpolation is adopted, the amplitude A32 at the time slot t2 is denoted by:A32=(A31*¾+A35*¼)
The phase θ32 is denoted by:θ32=(θ31*¾+θ35*¼)
In step 203, the amplitude and phase of the response H12 is obtained. Since the ratio of difference between the frequencies of the sub-carriers on the sub-channels C(1) and C(0) to that between the frequencies of the sub-carriers on the sub-channels C(1) and C(3) is 1:2, and a linear interpolation is adopted, the amplitude response of the sub-channel C(1) at the time slot t2 is denoted by:A12=(A02*⅔+A32*⅓)
The phase response is denoted by:θ12=(θ02*⅔+θ32*⅓)
However, in the previously described channel estimation, it is necessary to estimate the channel response of each sub-channel in each time slot at the receiver side, which requires a lot of resources. Besides, a large memory is required for storage of each estimated channel response, which increases the cost.